Method and apparatus for detecting corrosion



Oct. 24, 1950 P. s. WILLIAMS msmon AND APPARATUS FOR DETECTING CORROSION2 Sheets-Sheet 2 Filed April 5, 1947 PERM/1 MEN 7 M4 c/vz T 69:22am!cnsnvc on 7-015 nvc oan, raso,

mmrus Ion uncommcorurosroiv Y vII'I'IIOI) AND emotions 22 7 oFFrcsrumps.Willi-m,awn-i onic.mum-Iris mndnell parent Company,- a cerware amApril 5, must. No. new

.1 I I Thepresentinventionisdirectedtoametbod and apparatus fordetecting corrosion in pipes;

lloreparticular1y.itisdirectedtosuchamethod and apparatus peculiarlyadapted for detecting mosion in pipe which are sunk in the earth.

The principal object of the present invention is theprovision of amethod and apparatus by the employment of which it is possible to detectdefects in pipe due to corrosion or other causes which are otherwise notreadily detectable.

Another object of the present invention is the provision of a method andapparatus of the type indicated by the employment of which it ispossible to produce a log of a borehole casing, or other pipe situatedin a. borehole. revealing the location of any pits or holes orotherdefects in the buried pipe.

A more specific object of the present invention is the provision of amethod and apparatus of the type indicated characterized by the factthat the magnetic properties of a pipe are measured in such a mannerthat deformities in the pipe of the character of pits and holes producea readily distinguishable signal on a record.

An additional object of the present invention is the provision of anapparatus which may be lowered through a buried casing or pipe and whichcontains a magnetic scanner capable of producing an electrical signaland which also ineludes connections with the surface whereby saidelectrical signal may be recorded at the surface.

Other objects and advantages of the present invention will appear fromthe following detailed description and the accompanying drawings, inwhich; v

Fig. 1 is a front elevation partly in section of one embodiment of thepresent invention shown in position in a borehole;

Fig.2isaplanviewofonetypeofmagnetic scanner according to the presentinvention;

Fig.3isaplanviewofamodiiiedtypeofmagnetic scanner according to thepresent invention; and

Fig.4isaplanviewofarecord stripofthe wpe obtainable by logging a casingaccording to the present invention.

Referring to Fig. 1 in detail. numeral l designates a casing in aborehole, the surface of the ground being indicated by nrnneral 2.Arranged inside the casing is a cartridge or bomb 3 which is preferablyconstructed of a thin-walled. nonmagnetic material. The bomb is providedwith a bellows type spout 4 having a stoppered opening I for fllling'thebomb with a liquid such as kerosene. The purpose of the bellows typespout is to compensate for the pressure diii'erential between the insideand outside of the bomb whereby the walls need not have suillcientmechanical strength to withstand the borehole pressure.

Suitably mounted in the bomb 3 an electric l Claim. (Cl. 175-183) motor0 driven by 2 batteries I. 'Theshaftl of the motorcarriesat its freeendamagneticscannerlonwhichiswoundacoillltheendsof which are connectedto slip rings 'll mounted on the shaft I. The slip rings are connectedby conductors l2 to a recording apparatus at the surface, including anamplifier lland a recorder I4 which may be either of the pen type or thegalvanometer type. It will be imderstood that the conductors i2 arearranged inside a cable II from which the bomb is suspended. This cableat the surface is threaded over a measuring wheel II and is wound on adrum II. The conductors areconnectedtotheshaftofthedruminthe usualmanner and a slip ring arrangement is provided for connecting theconductors to the amplifler l3- All this surface equipment is customaryin various types of well logg ng operations and is, therefore, notdiscussed in greater detail. Such equipment usually includes means forsynchronizing the movement of the record strip with the movement-of theborehole bomb.

Extending from the bottom of the bomb is a rod I! having a stop 2| atits lower end. A collar 2| carrying three or more pivoted arms 22 isslidingly arranged on the rod l9. At the outer end of each rod is aroller 23 which is spring pressed against the casing by a spring 26.Links 28 are pivoted to the arms 22 and to a lower collar 26. Thisassembly of elements constitutes a self-adjusting centering device. Asimilar arrangement (not shown) is provided at the top of the bomb.

Referring to Fig. 2. the bomb casing is omitted in order that theprinciple involved may be better ilustrated. The shaft 8 of the motorcarries a T-shaped member, the cross portion 21 of which is arcuate inshape to conform to the shape of the interior of the casing. Thiscross'member isv composed of soft iron or other magnetic material andconstitutes, the south pole of a magnet. The free end 28 of the leg ofthe T is also composed of soft iron or other magnetic material. Thisfree end constitutes the north pole of the magnet and terminates a shortdistance away from the casing. This means. of course, that the wholeT-shaped member fits snugly in the bomb 3 which in turn is of across-sectional area closely approximately that of the interior of thecasin': whereby the distance between the poles of the magnet and thecasing to bestudied is little more than the wall thickness of the bomb.

Between the two pieces of soft iron there is interposed a strongpermanent magnet 29. A suitable material for use in this magnet isAlnico 5". A coil lll iswound around the north pole of the assembly inthe manner shown. 7

As the bomb is lowered in the borehole the scanner is continuouslyrotated. This scanner sets up a magnetic field which includes theeasing. The coil II is in this magnetic field. Where the casing isdeformed as by pits 3! resulting from corrosion, the magnetic field ismodified. The changing of the intensity of the magnetic field due tosuch deformities sets up an induced E. M. I". in the coil II which istransmitted to the surface and there amplified and recorded. If thecasing is sound the record will be substantially a straight line. Anychange in the magnetic field, however, due to deformities as heretoforeindicated. will produce peaks or humps on the record.

It may be observed that it is desirable to maintain the impedance of thecoil ll fairly low for reasons connected with pick-up of electricalnoise in the cable leading to the surface. On the other hand, because ofcable resistance, the coil impedance cannot be too low. The actualmakeup of the coil, therefore, must be a compromise, but is not toolimited. A coil having 200 turns of fairly thick copper wire giving aresistance of 60 ohms for the total coil has been found satisfactory. Itwill be apparent, however, that the actual specifications of the coilcan be varied depending on various factors.

In operating the device the rate of travel of the bomb along theborehole can be regulated in accordance with the detail desired. Aconvenient procedure is to lower the bomb through the casing ratherrapidly, say at a rate of about 50 ft./min. This will produce a recordon which sections of the casing containing deformities will beindicated, but in which little detail of the degree of deformity or itsexact location will be given. The bomb can then be moved upwardlyrapidly past those sections of easing which appear to be uniform, andslowly, say at a rate of about ft./min., past those sections which wereindicated on the down trip to require more detailed investigation Therate at which the scanning device is rotated can also be variedsomewhat. It will be appreciated that since the signal is produced bythe rate of change of the magnetic field the higher the rate of rotationthe more rapid will be this rate of change and, therefore, the largerthe signal. A rotation rate of 100 R. P. M. has been found to besatisfactory.

The modification shown in Fig. 3 differs from that shown in Fig. 2 in animportant respect. It will be observed that in the modification shown inFig. 2, the lines of force travel around the entire periphery of thecasing. In the modification shown in Fig. 3 the field of force isconfined to the area adjacent to the north and south poles of the magnetwhich are disposed close to each other. This means that the deformity inthe easing which comes within this field of force will have a greatereffect thereon than will the same deformity in the modification shown inFig. 2.

In Fig. 3 curved arms II and II are composed of soft iron or othermagnetic material. The rear ends of these arms are secured to apermanent magnet 33 of the character heretofore indicated. The coil IIin this case is wound on both arms in the manner shown. The free ends ofthe arms are spaced apart any desired distance depending upon the sizeof the deformities in be detected. This distance may vary from about Vto 2 or 3" or more, if desired.

In Fig. 4 is shown the type of record which maybeexpectedinthelosgingofaeasing. This record is in the form of a traceit on a strip of paper 8|. The large kicks It on this record in- 4dicate the casing Joints. The small kicks 81 indicate deformities whichin the ordinary case will be pits resulting from corrosion either on theinside or outside of the casing. This type of record is produced with afairly high rate of travel of the scanner along the casing with theresult that the deformities do not show up in any considerable detail.This record would indicate that the particular sections showing thekicks ll should be studied more carefully with a slower rate of travelof the bomb. The presence of the kicks 3. on the record is very helpfulsince it enables the observer to determine more accurately the depth atwhich deformities are indicated.

It is to be understood that the existence of magnetic lines of force inthe earth is taken into account in the design of the present invention.It is by reason of the possible existence of such lines and theirpossible magnetization of portions of a casing that a strong magnetcomposed of material such as Alnico 5 is used. Such a magnet sets up amagnetic field sufficiently stron so that the existence of any naturalmagnetic lines of force will constitute so slight a percentage of thetotal magnetic field as to have no noticeable effect on it.

The specific embodiment of the present invention described hereinrelates to the use of the scanner in the examination of well casing. Itwill be understood that a properly proportioned scanner can also beemployed for the study of well tubing. Likewise the apparatus can bereadily adapted for use in surface pipe lines. In this case where theexterior of the pipe lin is exposed and can be readily examined, asimilar device may be moved along the exterior for detecting flaws ordeformities on the inner surface of the pipe.

The nature and objects of the present invention having been thusdescribed and illustrated, what is claimed as useful and is desired tobe secured by Letters Patent is:

An apparatus for detecting deformities in a pipe comprising a scanningdevice adapted to be moved along the inside of the pipe, said scanningdevice comprising a pair of curved arms of ma netic material lying in aplane perpendicular to the longitudinal axis of said pipe, a stronpermanent magnet placed between and secured to one end of each of saidarms, the free ends of the arms being curved in toward each other insaid plane so as to terminate in close spaced relation to each other, acoil wound about at least one of said arms, means for rotating saidscanning device about the longitudinal axis of said pipe in said plane,means for maintaining the termini of said free ends of said arms inclose spaced relation to the wall of said pipe, and means for detectingvoltages generated in said cell.

PHILIPS. WILLIAMS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,179,240 Breitenstein Nov. 7,1939 2,207,592 Lenk July 9, 1940 2,255,053 Gunn Sept. 9, 1941 2,259,904MeNamee et a1. Oct. 21, 1941 2,308,159 Drummond et al. Jan. 12, 19432,359,894 Brown et al. Oct. 10, 1944

